Information can be encoded on the optical carrier signal in a fiber optic communication network using an electro-optic (EO) modulator. In these modulators, nonlinear optic (NLO) materials having second-order NLO activity are necessary to effect modulation of the light signal.
Polymeric thin films containing asymmetrical chromophores, in a polar orientation, have been under investigation for their second-order NLO, piezoelectric, and/or pyroelectric properties for many years. This area of research has yielded several types of versatile polymers useful for applications such as optical second harmonic generation and EO modulation of optical signals. The following information is provided as a brief overview of technology relevant to these NLO materials. For a more detailed discussion involving this technology please refer to U.S. Pat. No. 5,247,055 issued to Stenger-Smith et al., Sep. 21, 1993; U.S. Pat. No. 5,520,968, issued to Wynne et al., May 28, 1996; and the book, Polymers for Second-Order Nonlinear Optics, G. A. Lindsay and K. D. Singer, Eds., Am. Chem. Soc. Advances in Chemistry Series 601, Washington, D.C., 1995, and references therein.
Organic polymeric thin films for photonic applications has been a rapidly evolving area of research for over 15 years. One class of materials within this field, nonlinear optical polymer (NLOP) films, has potential for breakthroughs in low cost integrated devices for the telecommunication and data-communication industries. Key components of this new technology are EO waveguides made from second-order NLOP films. These waveguides can switch optical signals from one channel to another and can modulate the phase or amplitude of an optical signal by means of applying an electric field across the channel.
NLO materials used in EO devices have, in general, been inorganic single crystals such as lithium niobate (LiNbO3) or potassium dihydrogen phosphate (KDP). More recently, NLO materials based on organic molecules, and in particular polar organic chromophores have been developed.
Organic NLO materials have a number of potential advantages over inorganic materials. First, some organic NLO materials have a higher electro-optic coefficient (r33). [See U.S. Pat. No. 6,348,992; Chemistry of Materials 2002, volume 14, pp. 4669–4675; and Advanced Materials 2002, volume 14, pp. 1763–1768 and references therein.] Second, the low index of refraction of organic materials (e.g., 1.3 to 1.7) compared to that of inorganic materials (e.g., 2.1 to 3.5) leads to lower optical loss due to less reflection when coupling the optical modulator to an optical fiber. Third, the organic materials can be easily fabricated into integrated device structures when used in polymer form.
EP 218,938 and U.S. Pat. No. 4,859,876 issued to Dirk et al., Aug. 22, 1989 have used an approach of incorporating NLO-active guest chromophores into amorphous polymer host matrices by blending. Such polymeric materials have the advantages of being easily fabricated into thin films suitable for integrated optical devices. In order for the film containing organic chromophores to have NLO activity, the chromophores must be given a polar orientation to achieve a non-centrosymmetric alignment. Such alignment is usually achieved by the application of an electric field across the film thickness while the temperature of the polymeric blend is above or near its glass transition temperature (Tg). The polymer is then cooled with the field on to lock the oriented molecules in place. EP 218,938 discloses a number of polymer host materials, including epoxies, and many types of chromophores which have NLO activity including azo dyes such as Disperse Red 1. It is known that an NLO active material such as azo dye Disperse Red 1, (4-[N-ethyl-N-(2-hydroxyethyl]amino-4-nitro azobenzene), may be incorporated into a host by simply blending the azo dye in a thermoplastic material such as poly(methylmethacrylate), as described U.S. Pat. No. 4,859,876.
While the doped polymer approach offers some advantages over organic and inorganic crystals, the approach has a number of problems. First, the stability of the NLO activity over time of such materials has been shown to be less than desired.
In addition, the guest NLO chromophores blended in a polymer plasticize the polymer host matrix, lowering the glass transition temperature (Tg). Lowering the polymer Tg reduces the thermal stability of the NLO film or NLO medium. Near the Tg, segments of the polymer become mobile and the NLO active guest chromophores, which were oriented by electric-field poling undergo orientational relaxation. Once orientational relaxation has occurred, the NLO medium exhibits no NLO activity.
A third problem with the guest-host NLO polymers are the often-found poor solubility of the NLO chromophore in the host matrix. The guest chromophores can aggregate at relatively low doping levels (5–20 percent w/v). Such aggregates scatter light and reduce the transparency of the waveguides to unacceptable levels.
One method to improve the orientation stability is to attach the chromophore to a polymer backbone. There have been many examples of this in literature. U.S. Pat. No. 5,208,299 issued to Bales, et al., May 4, 1993, concerns making arylhydrazones and polymerizing them with other commoners resulting in optically transparent polymers that exhibiting second-order NLO activity upon orientation. In addition, U.S. Pat. No. 5,208,299 discloses polymers comprising the NLO materials or medium having a relatively high Tg, resulting in high temperature stability. However, U.S. Pat. No. 5,208,299 does not provide the index tunability of the preferred embodiment of the present invention.
U.S. Pat. No. 5,288,816 issued to Inbasekaran, et al., Feb. 22, 1994, discloses NLO aminoaryl hydrazones as curing agents for epoxy resins, and as suitable monomers for polymeric compositions such as poly(amino ethers), polyimides, polyamides, and polyureas. Also disclosed are epoxy polymers or epoxy based polymers containing covalently bonded aminoaryl hydrazone moieties in the structure of the polymers exhibiting enhanced nonlinear optical activity and stability. In addition, similar to U.S. Pat. No. 5,208,299, U.S. Pat. No. 5,288,816 discloses an invention in which the polymers comprising the NLO materials have a relatively high Tg, resulting in high temperature stability of the NLO material. However, U.S. Pat. No. 5,288,816 does not disclose the index tunability of the preferred embodiment of the present invention.